DSL (digital subscriber line)
The foundation of the technology is the four-wire telephone cable that is standard throughout North America.
Until recently, only two wires were needed for voice calls. The other two were largely unused until the mid-90s, when explosive demand for Internet bandwidth led several firms to look beyond the 56K bit/sec limit on analog dial-up connections.
Early DSL standards entailed installing a signal splitter at the customer premises to separate the voice and data as well as a "DSL modem" to connect the user's PC to the data-stream. The two extra wires in standard telephone cable were employed to carry digital data to and from the local telephone office, which had to be located no more than 2 1/2 miles away. At the telco office, a digital subscriber line access multiplexer, or DSLAM, served as the connection between the individual DSL streams and the backbone network. Top speeds were 1.5M bit/sec, but at a much lower cost than a T-1 or ISDN connection.
As the technology has matured, the DSL playing field has gotten a lot more crowded, and a lot more complex. In fact, DSL is often known as "xDSL" to account for the many flavors of DSL that have arrived on the scene. A common variant is asymmetric DSL, or ADSL. Asymmetric refers to the unequal upstream and downstream speeds -- typically, you have much higher bandwidth for downloading than uploading. ADSL technology is aimed at home users and small offices that are mostly interested in high-speed access to Web content, and even streaming media. Download speeds can top 8M bit/sec, depending on the distance to the local telco office. Upload speeds are much slower - typically maxing out at 640 kilobits per second. As for cost, the faster the downstream DSL connection, the higher the monthly price.
Symmetric DSL, or SDSL, has equal upstream and downstream speeds of up to 1.5M bits/sec in both directions. Then there is VDSL, or very high bit rate DSL, a developing asymmetric flavor that can reach downstream speeds of 55M bit/sec, but only at a distance of up to 1000 feet from the local telco office. Beyond that, VDSL transmission rates drop dramatically, which severely limits its customer base.
Also gaining in popularity is DSL Lite, or G.Lite. This is yet another asymmetric variant, but what makes it different is that it does not require a splitter at the customer premises. The line is instead split at the telco office, which cuts down installation costs. Speeds range from 1.5M bit/sec to 6M bit/sec downstream, and about one-tenth those rates for upstream.
From DSL vs. Cable, an audio discussion of the two broadband technologies.
Additional resources
DSL research center
The latest DSL news, analysis and opinion from Network World Fusion.
Audio Primer: DSL vs. Cable
Listen to a seven-minute comparison of the two broadband technologies.
DSL Reports
See which providers offer DSL at specific locations, look up user comments on providers.
Comments:
broadband access systems
by anbu
What is the role of traffic management in broadband access systems?
Add a comment
